Hangers for slanted walls....

I would think that either security hangers or a top & bottom installation of zbar would be your preferred choices. With the zbar, reverse the bottom bar so that it faces up instead of down. This will lock it onto the wall.
 
WallBuddies will not help on a slanted wall.

I suggest using mending plates, screwed to the side rails on the back of the frame, and the wall. If the frame is big, use two pairs. When the installation is done, the frame is literally screwed to the wall, but the mending plates are hidden behind the frame. No special tools required.

The method is easier than it seems. If you want detailed instructions, let me have your email address and I'll send you a drawing.
 
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